Soil creep is the very slow movement of soil down a slope. The movement is so slow and broad in scope that it does not normally reveal itself by scars on the landscape, nor does it disrupt the formation of soil horizons. Soil creep differs from erosion in that creep involves a large mass of soil (probably the entire solum) moving en masse for a short distance, whereas erosion takes a smaller amount of soil and removes it from the area.
The slowness of soil creep makes it easy to ignore or overlook. The creep is therefore seldom mentioned and rarely measured. Nevertheless, its effect can be significant because they are cumulative over long periods. The persistence of soil movement makes it significant to sciences dealing with soils or landscapes. One centimeter of soil movement per year would add up to a meter in a century. Some soils may have moved several meters while man has been using them and possibly several kilometers during their total existence.
Two main difficulties must be overcome in measuring the rate and amount of soil creep. One difficulty is posed by the slow rate of movement. Either long times must be considered, or very precise measurements must be made. The second difficulty is the need for stable reference points from which the soil movement can be measured.
Some workers have measured the rate of movement of stones on the soil surface rather than that of the soil itself. One previous worker drove metal stakes through the soil into bedrock to serve as stationary reference points. Marked stones were placed on the soil surface and measured relative to the metal stakes. The stones moved downslope at rates ranging from a few millimeters per year on a 3.degree. slope to almost 70 millimeters per year on a 40.degree. slope. It was recognized that the stones moved faster than the soil because some stones were caught in cracks and moved more slowly than the others. This difference in rate of movement is at least partially dependent on the relative densities of the stones and the soil.
Other scientists have recognized the importance of density effects on rates of movement and prepared hollow steel cylinders and weighted them to match the soil density. The cylinders were buried at various depths in the soil and observation tubes were placed above them. The positions of the cylinders relative to fixed positions in the underlying chalk bedrock were measured with theodolites. This method required multiple measurements and a complex mathematical analysis to evaluate the movement. It was found that the movement was spasmodic and parallel to the steepest slope direction.
Scientists have also attempted to measure soil creep by burying paraffin rods in auger holes. The lower parts of the paraffin rods were embedded in compact glacial till and the upper parts were in the soil solum. The rods were dug up after a time, and their shapes indicated the amount and depth of movement that had occurred. A further device used to measure soil creep was a metal probe strip with several strain gages built into it. The strips were buried in the same manner as the paraffin rods. Wires from the strain gages led to the soil surface where an electric meter was attached when readings were to be taken.
Therefore, it is the principal object of the invention to provide an improved apparatus for detecting soil movement. A still further object of the invention is to provide an apparatus for detecting soil movement which measures soil creep simply and accurately.
A still further object of the invention is to provide an apparatus for detecting soil movement which is capable of measuring the rates of movement on lesser slope gradients than most of the prior art devices and methods.
A still further object of the invention is to provide an apparatus for detecting soil movement which eliminates the complex mathematical analysis which some of the previous apparatuses and methods have required.
A still further object of the invention is to provide an apparatus for detecting soil movement which does not interfere with normal tillage of the ground after the apparatus has been used to detect the soil movement.
A still further object of the invention is to provide an apparatus for detecting soil movement which is economical to manufacture, durable in use and refined in appearance.
These and other objects will be apparent to those skilled in the art.